1. Field of Invention
The present invention relates to the leadframe of a quad flat non-leaded package. More particularly, the present invention relates to a quad flat non-leaded package having a leadframe that includes a thinner connected region.
2. Description of Related Art
Semiconductor packages can be roughly divided into pin through hole (PTH) type and surface mount type. The pin through hole (PTH) type is a type of package which leads can be inserted into a plurality of pre-drilled holes of a circuit board. Typical pin through hole (PTH) type package includes the dual in line (DIP) package and the pin grid array (PGA). The surface mount type package requires printing a solder paste over the corresponding mounting pads of a circuit board followed by a solder reflowing process. Conventional surface-mount packages include small outline package (SOP), quad flat package (QFP) and ball grid array (BGA).
Due to its limitation of pin count, the dual in line (DIP) type package is gradually phased out. In turn, the pin grid array (PGA) package is difficult to assemble and has surface area that is larger than other conventional packages, while the production cost is generally high. Among various types of packaging structure, the techniques for fabricating the quad flat package are currently mature.
The leads of a quad flat package extend from the periphery of the integrated circuit (IC) to four horizontal directions, while the pitch between the leads also shrinks from a former value of 1.27 mm to about 0.3 mm. Most of the quad flat packages are made of plastic. Currently, a quad flat package can have at most 300 external leads. At present, quad flat type package is employed to package logic IC and low-to-moderate level micro devices. The U.S. Pat. No. 5,942,794 discloses a quad flat non-leaded (QFN) package. xe2x80x9cNon-leadedxe2x80x9d means that the quad flat package does not have outer leads. Hence, signal transmission path is shortened and a signal decay thereby can be reduced.
The plastic molding process of a conventional quad flat non-leaded package can be roughly classified into strip mode (in strip mode, the leadframe of various packaging units are connected from one dimension) and array mode (in array mode, the leadframe of various packaging units are connected from two dimensions). No matter how various packaging units are connected together, the standard procedure for manufacturing the quad flat non-leaded package includes die bonding, wire bonding, molding and singulation.
FIG. 1 is a top view showing a leadframe of molded array type used to form an array of conventional quad flat non-leaded packages. As shown in FIG. 1, the leadframe 109 has a sheet structure. The leadframe 109 has a plurality of packaging units 111. Each packaging unit 111 can be divided into two major regions: a lead portion and a flat portion. The lead portion includes only internal leads 100. The flat portion includes a die pad 106 for holding a die 102 (the die 102 is shown in FIG. 2). To facilitate the automation of the production, the leadframe 109 may include side rails (not shown) for linking various packaging units and pilot holes (not shown) for an in-process alignment. The leadframe 109 also has tie bars 103 for fixing the die pad 106 in position relative to the leadframe 109. A dam bar 108 is used to connect various leads 100 with the side rails. The dam bar 108 also serves to prevent an extrusion of the plastic compound from occurring during the molding process. The dam bar 108 is also directed to linking various packaging units 111 inside the leadframe 109. The outermost packaging unit 111 is surrounded by the dam bars 108 and side rail.
FIG. 2 is a schematic cross-sectional view showing the leadframe in FIG. 1 after die attaching, wire-bonding, and encapsulating but before singulation. As shown in FIG. 2, the conventional quad flat non-leaded package comprises the die pad 106 and the plurality of inner leads 100 surrounding the die pad 106. The backside 102b of the die 102 is attached to the upper surface 106a of the die pad 106 using epoxy 101. Bonding pads (not shown) on the active surface 102a of the die 102 is electrically connected to the upper surface 100a of the inner leads 100 through the gold wires 104. The die 102, the gold wires 104, the upper surface 106a of the die pad 106, the upper surface 100a of the inner leads 100 are encapsulated by the molding compound 105. Only the lower surface 106b of the die pad 106 and the lower surface 100b of the inner leads 100 are exposed. Utilizing the exposed lower surface 100b of the inner leads 100, the package is connected to an external printed circuit board (not shown).
After encapsulating, the singulation process is conducted. The purpose of singulation is to separate the molded array package into individual package units 111. A basic criterion in singulation is the clean removal of excess plastic and dam bar material to form a smooth package. However, the hardened molding compound and the leadframe material have different physical properties. This may cause some problems in the cutting process because the cutting blade may distort the leadframe when encountering a non-uniform material.
There are three major drawbacks in a conventional quad flat non-leaded package, including:
1. Penetration of moisture: as shown in FIG. 2, due to distortion of the inner leads 100 in a quad flat non-leaded package after blade cutting, gaps are often formed between the inner leads 100 and the molding compound 105. Hence, moisture can easily penetrate into the package.
2. Outward-plugging of leads after singulation causes a drop in reliability: position of the cutting street 113 is indicated by dashed lines in FIGS. 1, 2 and 3. The cutting street 113 having a width A is the location where a cutting blade 107 (shown in FIG. 3) needs to pass through.
FIG. 3 is a cross-sectional view showing a cutting blade on top of the cutting street between two neighboring packages in an array of packages. As shown in FIG. 3, the cutting blade 107 is posed on top of the cutting street 113 at the junction of two packaging units 111.
In the conventional quad flat non-leaded package, the inner leads 100 have a uniform shape. Hence, some of the inner leads 100 can be easily pulled out from the molding compound 105 by the cutting blade 107 tension while cutting. FIG. 4 is a cross-sectional view of two quad flat packages after singulation showing some partially pulled inner leads.
3. Dam bar residues on packages is caused by a blade misalignment during singulation. As shown in FIG. 1, the cutting street 113 has a width A, and the leadframe 109 has a thickness t1 as shown in FIG. 2. According to general design rules, dam bar should have a minimum width w1 of about 0.7t1. Consequently, maximum misalignment tolerance of the cutting blade is Axe2x88x92w1/2=Axe2x88x920.7t1/2=0.5Axe2x88x920.35t1. In general, width A of the cutting street 113 is about 0.3 mm (width of the cutting blade 107) and the leadframe 109 has a thickness t1 of about 0.2 mm. Following standard design rule, the dam bar thus should have a width 0.7xc3x970.2=0.14 mm. Hence, the misalignment tolerance of the cutting blade is (0.3xe2x88x920.14)/2=0.08. Such a small tolerance of the cutting blade often results in the attachment of some dam bar residue on the package leading to poor package products. FIG. 5 is a top view showing dam bar residue on singulated quad flat non-leaded packages. As shown in FIG. 5, a portion of the dam bar material is attached to the package after singulation. Hence, a lower production yield is obtained.
One object of the present invention is to provide a leadframe having a thinner connected region that can lower the cutting force on a blade required to perform singulation.
Another object of the present invention is to provide a leadframe having a thinner connected region that can reduce the attachment of dam bar residue to a singulated package due to the blade misalignment.
Yet, another object of the present invention is to provide a leadframe having a thinner connected region that can strengthen the adherence between the leads and the molding compound and thus increase overall reliability of the package.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a leadframe structure for supporting a die within a quad flat non-leaded package. The leadframe structure includes a plurality of packaging units. Each packaging unit has a plurality of leads surrounding a die pad. There is a dam bar between neighboring packaging units. The dam bar and the junction between the leads of neighboring packaging units have a section that is relatively thinner than regions elsewhere.
According to this invention, the leadframe is half-etched to form a thinner connected section along the cutting street.
By reducing the thickness of the cutting street, the depth of cutting for a cutting blade during singulation are substantially reduced. Ultimately, the probability of pulling any lead out of the matrix of plastic material that encloses the leadframe is reduced. Furthermore, the reduction of thickness in the local lead region facilitates the formation of a step structure close to the tip of the leads, thereby strengthening the adherence between the leads and the molding compound. In addition, the thinner connected regions on the leadframe reduce the width of the dam bar. Hence, a larger cutting blade misalignment can be tolerated. Overall, a higher singulation yield can be attained.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.